A liquid crystal display cell generally includes a first transparent plate covered with a counter-electrode and a second plate, also transparent, covered with addressing lines and columns, and inactive matrix screens with a matrix of control transistors and pixels. These two plates are kept apart by spacers and form a volume filled with liquid crystals.
The counter-electrode is generally constituted by a fine film of tin or indium oxide (ITO). It may be completed by an optical mask or by colored filters.
FIGS. 1 and 2 illustrate a conventional method for forming such a cell, especially showing the means for forming the electric connection between the counter-electrode and the control circuits placed outside the cell.
In order to form this cell, a first plate P1, known as a counter-electrode plate, is made for example of glass, on which the counter-electrode CE is placed.
In addition, a second plate P2, known as an electrode plate is made for example of glass, on which a network of addressing lines L and columns C are placed, as well as a matrix of pixels Px. The electrical connections are carried out at the line and column ends. This second plate may be embodied in two masking levels, as described in the document FR-A-2 533 072.
So as to form the electric connection of the counter-electrode CE, rather than provide this connection directly on the counter-electrode plate P1, it is preferable to establish this connection on the electrodes plate P2 with the other connections. With this purpose in mind, special contacts 20 are provided disposed, for example, at the four corners of the plate of electrodes and made of a good conductive metal, such as aluminium. Of course, this means that it is essential to establish a link between the counter-electrode CE and these contacts 20. Conductive blocks 12 made, for example of silver paste, are thus disposed on the counter-electrode at locations corresponding to the locations of the contacts 20, that is in the variant shown, at the four corners of the counter-electrode.
So as to avoid any electrochemical phenomena and also any possible short-circuits between the plates, the plate of electrode P2 is generally coated with a nonconducting passivation film. For example, this may be a 300 nm film of silicon nitride. As regards this film coating the contacts 20, in order to establish the electric link between the counter-electrode CE and the contacts 20, it is necessary to pierce this film at the locations corresponding to the contacts. FIG. 2 shows a section of the zone for establishing the electric link between the counter-electrode CE and a contact 20 through an opening 24 pierced in the passivation film 22 and by means of the silver paste block 12.
The openings 24 are obtained by means of a complete photolithography operation with resin tapping, alignment, insolation and resin development, etching of the passivation film and elimination of the resin.
Once the two plates P1 and P2 have been formed and the passivation film etched at the appropriate locations, the method for forming the cell is continued by placing an alignment film on the plate of electrodes P2 and then by pulverizing the spaces. An alignment film is placed on the counter-electrode plate P1, a polymerisable glue cord 10 is serigraphed and then silver paste blocks 12 are placed by means of a syringe at the appropriate locations. After the two plates are assembled, the unit is annealed so as to polymerize the glue and the silver paste.
If this method provides satisfactory, it is clear that the operations for etching the openings 24 in the passivation film 22 heavily penalize the method which, without this additional etching, would remain simple since only two photoetchings are required, at least in the variant described in the aforesaid document FR-A-2 533 072.
The present invention is able to resolve this drawback by providing a method able to retain the passivation film without it nevertheless being necessary for any additional photolithography so as to establish the electric link between the counter-electrode and its contacts.